PECULIAR PHOTOCONDUCTIVITY IN NONLINEAR GaAs PCSS

Photoconductive semiconductor switches (PCSSs) are considered as promising device s for high power applications. Since picosecond optoelectronic switching in silicon was published in 1975, especially from 1977, in which Si was replaced by GaAs, PCSSs have been significantly used in pulsed power technology, such as high-power ultra wideband microwave source and compact pulsed power generator. Since GaAs is a direct bandgap semiconductor, PCSSs fabricated from GaAs can operate in linear and nonlinear mode. In the nonlinear mode, there is a special effect termed lock-on, which means that after the trigger laser pulse has ended, the voltage across the PCSS drops to a constant value and stays on, instead of dropping to zero. This paper reports the peculiar photoconductivity in nonlinear GaAs photoconductive semiconductor switch. The PCSS with a gap of 18 mm was fabricated from semi-insulating GaAs and triggered by 1064 nm laser pulse. The PCSS operated in linear mode at bias voltages of 4 kV and 6 kV, because the waveform of photocurrent is similar to that of laser pulse. As the bias voltage increased, the FWHM of photocurrents also increased, the PCSS transited from linear mode to nonlinear mode. The switch behaved nonlinear at a bias voltage of 10 kV, with the obvious lock-on in photocurrent waveform. However, with a bias voltage above 10kV, the lock-on current increased instead of staying on a constant value, and as the bias voltage increased further, the slope of photocurrent also increased. The experimental results with different bias voltage are presented, and the peculiar photoconductivity is discussed.